Life enhancement of ring assembly in semiconductor manufacturing chambers

ABSTRACT

The present invention generally relates to a ring assembly that may be used in an etching or other plasma processing chamber. The ring assembly generally includes an inner ring body having a top planar surface and a bottom planar surface, and an outer ring body having a top surface, a bottom surface substantially parallel to the top surface, and an inside surface that extends between the top surface and the bottom surface, the inside surface having a roof covering a portion of the inner ring body when the inner ring body is disposed adjacent the roof, wherein the inner ring body can be flipped into a different position so that a portion of the inner ring body that is not covered by the roof provides a substantially planar surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/013,772, filed Jan. 25, 2011, which claims benefit of U.S.Provisional Patent Application Ser. No. 61/298,854, filed Jan. 27, 2010,which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to a ring and ringassembly for an etching or other plasma processing chamber.

2. Description of the Related Art

In semiconductor processing chambers, substrates undergo variousprocesses such as deposition, etching and annealing. During some of theprocesses, the substrate is placed onto a substrate support such as anelectrostatic chuck (ESC), for processing. In an etch process a ring maybe placed around the substrate to prevent erosion of the areas of thesubstrate support that are not covered by the substrate. The ringfocuses the plasma and positions the substrate in place.

Rings are usually made of quartz material and are highly consumed in theetch process as they are exposed to etching gases and/or fluids. Therings are etched during wafer processing and need to be replaced after aset of substrates (wafers) are processed. The rings that aresignificantly eroded are usually replaced during wet clean. Wet cleaningis a process for removing etching residues using a cleaning chemical,and some of the cleaning chemicals may remain in the chamber afterchamber cleaning. Therefore, the rings may be exposed to and eroded bythe cleaning chemical. Thus, significantly reducing the service life ofthe ring. In some situations, a ring may need to be replaced after everywet clean, which leads to wastage of expensive quartz material.

Thus, there is a need in the art for a ring with an improved servicelife.

SUMMARY OF THE INVENTION

The present invention generally relates to a ring assembly that may beused in an etching or other plasma processing chamber. A ring assemblyas described herein is designed to increase the useful life of the ringas compared to conventional rings. The ring assembly has at least twodistinct pieces, also referred to as bodies. An inner ring body willcorrespond to a location where the majority of erosion occurs duringuse. The inner ring body of the ring assembly can be flipped and usedagain until both sides have eroded beyond their useful life.Collectively, the two bodies generally have the shape of a single piecering, but service life of the ring assembly has a longer service lifethan a conventional single piece ring.

In one embodiment, a ring assembly is provided. The ring assemblygenerally includes an inner ring body having a top planar surface and abottom planar surface, and an outer ring body having a top surface, abottom surface substantially parallel to the top surface, and an insidesurface that extends between the top surface and the bottom surface, theinside surface having a roof covering a portion of the inner ring bodywhen the inner ring body is disposed adjacent the roof, wherein theinner ring body can be flipped into a different position so that aportion of the inner ring body that is not covered by the roof providesa substantially planar surface.

In another embodiment, a ring assembly is provided. The ring assemblygenerally includes an outer ring body, comprising a top surface and abottom surface substantially parallel to the top surface, an outsidesurface extending between the top and bottom surfaces, an inside surfaceextending between the top and bottom surfaces, the inside surface havinga notch, and the notch having a roof projecting from an upper portion ofthe outer ring body, a first and second annular extensions extendingfrom the bottom surface of the outer ring body, wherein the first andsecond annular extensions are parallel and radially spaced from eachother, and a third annular extension extending from the outer ring bodyin a direction perpendicular to the first and second annular extensions,the third annular extension forming part of the inside surface andhaving a surface slanted relative to the top surface and the bottomsurface of the outer ring body at an angle above zero, and an inner ringbody disposed radially inward of the outer ring body, the inner ringbody comprising a top surface abutting and projecting inward of thethird annular extension, and an outside diameter surface disposed belowthe third annular extension and abutting the outer ring body.

In yet another embodiment, a ring assembly is provided. The ringassembly generally includes an outer ring body having a top surface, abottom surface opposite the top surface, an outside surface extendingbetween the top and bottom surfaces, and an extension extending from theouter ring body in a direction away from the outside surface, whereinthe extension has a surface slanted relative to the top surface at anangle above zero, and an inner ring body being surrounded by the outerring body, wherein the inner ring body is symmetrically nestable againstthe outer ring body in two orientations, the nested inner ring body andouter ring body having the same combined sectional profile when theinner ring body is in either flipped orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1A is a schematic, cross sectional view of an exemplary substratesupport with a conventional single piece ring disposed in a processchamber.

FIG. 1B is a schematic, cross sectional view of the ring of FIG. 1A.

FIG. 1C is a schematic illustration showing erosion profile of the ringof FIG. 1A.

FIG. 2A is a schematic, cross sectional view of an exemplary substratesupport having a ring assembly according to one embodiment of theinvention.

FIG. 2B is a schematic, cross sectional view of the ring assembly ofFIG. 2A.

FIG. 3A is a schematic illustration showing erosion profile of the ringassembly according to one embodiment.

FIG. 3B is a schematic, cross sectional view of the ring assembly afterthe inner ring has been flipped according to one embodiment.

FIG. 4 is a schematic, cross sectional view showing spacing betweeninner and outer rings of the ring assembly according to one embodiment.

FIG. 5A is a schematic cross sectional view of an outer ring of a ringassembly according to one embodiment.

FIG. 5B is an enlarged view of a portion of the outer ring illustratedin FIG. 5A.

FIG. 6A is a schematic cross sectional view of the inner ring of a ringassembly according to one embodiment.

FIG. 6B is an enlarged view of a portion of FIG. 6A.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

DETAILED DESCRIPTION

The present invention generally relates to a ring assembly that may beused in an etching or other plasma processing chamber. The ring assemblymay be made at least in two distinct pieces, for example, an inner ringbody and an outer ring body which nest together. The inner ring body hasa geometry that allows the ring assembly to maintain a single sectionalprofile when the inner ring body and outer ring body are nestedtogether, no matter which side of the inner ring body is facing theouter ring body. The inner ring body is reusable in that if one surfacebecomes eroded, the inner ring body may be flipped over and used againwith the outer ring body until the outer ring body and/or both sides ofthe inner ring body have eroded beyond their useful life. Collectively,the two bodies generally have the shape of a single conventional piecering, but the service life of the ring assembly is longer than aconventional single piece ring.

FIG. 1A is a schematic, cross sectional view of an exemplary substratesupport 100 with a conventional single piece ring 104 disposed in aprocess chamber 128. While not discussed here in detail, the substratesupport 100 is typically disposed in a plasma processing chamber, suchas an etching chamber. The process chamber 128 may be utilized alone or,as a processing module of an integrated semiconductor substrateprocessing system, or cluster tool. The substrate support 100 isdisposed below a showerhead 120, which is used to supply various gasesinto the process chamber 128. The substrate support 100 may be biased bya DC power supply (now shown). An RF power source 126 may optionally becoupled to the substrate support 100 through a matching network 122. Theshowerhead 120 may be coupled to an RF power source 132 through amatching network 124. The interior of the process chamber 128 is a highvacuum vessel that is coupled through a throttle valve (not shown) to avacuum pump 134. In operation, the substrate is placed on the substratesupport 100 and the chamber interior is pumped down to a near vacuumenvironment. One or more gases are introduced from the gas panel via theshowerhead 120 into the process chamber 128, and ignited into a plasmain the process chamber 128 by applying the power from the RF powersource 132 to the showerhead 120 disposed on the substrate support 100and/or applying RF power from the RF source 126 to the substrate support100 while applying the power from the bias source to bias the substratesupport 100. The formed plasma may be used to etch a feature in asubstrate during processing and then pumped out of the process chamber128 through the vacuum pump 134. It is understood that other componentsof the process chamber 128 have been omitted for the sake of brevity.

The substrate support 100 generally includes an electrostatic chuck(ESC) 102, a conventional single piece ring 104, a cathode 106 toelectrically bias the ESC 102, an insulator pipe 108, a pedestalinsulator 110, and a pedestal support 112. The insulator pipe 108 andthe pedestal insulator 110 function to electrically isolate the chamberwalls and the substrate support 100, respectively, from the electricalbias applied to the ESC 102. In the embodiment shown in FIG. 1A, theconventional single piece ring 104 is a single piece ring that rests onthe ESC 102 and the insulator pipe 108. A substrate 144, when placedonto the substrate support 100, will rest on the ESC 102 and besurrounded by the conventional single piece ring 104.

FIG. 1B is a schematic, cross sectional view of the conventional singlepiece ring 104 of FIG. 1A. The conventional single piece ring 104 has anupper surface 130 which is exposed to the plasma during processing. Theconventional single piece ring also includes a seating area 114 that hasa top surface 116 that is substantially level with a substrate 144placed on the ESC 102. The existing “single piece” ring design istypically subjected to maximum erosion near a wafer seating area 114 dueto exposure to a plasma etching process. FIG. 1C shows an approximateerosion profile 118 of the top surface of the conventional single piecering 104 near an end of the ring service life, with the original uppersurface 130 shown in phantom for comparison. As shown in FIG. 1C, theseating area 114 has eroded significantly more than the remaining areasof the conventional single piece ring 104 and has eroded non-uniformly.As the seating area 114 no longer has its original substantially planarshape, substrate positioning and processing may be adversely impacted.The entire ring 104 is typically replaced at this time, which results inwastage of quartz material.

In plasma etch chambers, the conventional single piece ring 104 protectsthe ESC from exposure to plasma during processing. Since the ring 104also focuses the plasma, the ring 104 is usually made of quartz and ishighly consumable during processing. In order to extend the life of therings, a ring assembly of the present invention is designed for theleast cost of manufacture and allow multiple usage to thereby reduce thecost of tool ownership (CoO) to customers.

FIG. 2A illustrates a schematic, cross sectional view of an exemplarysubstrate support 200 having a ring assembly according to one embodimentof the present invention. While not discussed here in detail, thesubstrate support 200 is typically disposed in a plasma processingchamber, such as an etching chamber as illustrated in FIG. 1A, which hasbeen omitted for the sake of brevity. The substrate support 200generally includes an ESC 202, a ring assembly 230, a cathode 208, aninsulator pipe 210, a pedestal insulator 212 and a pedestal support 214.

FIG. 2B is a schematic, cross sectional view of the ring assembly 230 ofFIG. 2A. The ring assembly 230 of the present invention generallyconsists of two ring bodies, i.e., an inner ring body 206 and an outerring body 204, which are positioned on the electrostatic chuck 202during processing. Similar to the conventional single piece ring 104discussed with reference to FIGS. 1A-C above, most of the erosion takesplace on the inner ring body 206 near the substrate seating area B,causing the top surface of the inner ring body 206 to no longer beplanar (see erosion profile 306 and eroded seating surface 352 in FIG.3A). However, once the eroded inner ring body 206 is flipped as shown inFIG. 3B, the substrate seating area B is once again substantiallyplanar. Thus, rather than replacing an entire single ring, the innerring body 206 may be flipped over and reused with the outer ring body204. Flipping of the inner ring body 206 doubles the service life of theentire ring assembly 230 relative to the conventional single piece ring104.

In one embodiment, the inner ring body 206 is designed such that aportion of the surface (area A as shown in FIG. 2B) is covered by theouter ring body 204 and hence prevents exposure to plasma. As a result,the area A remains substantially planar even as the area B erodes fromplasma exposure. Since area A remains planar, this surface (i.e., areaA) provides a planar bottom surface to the inner ring body 206 afterbeing flipped over, as shown in FIG. 3B. After a wet clean cycle, theinner ring body 206 can be flipped over along an axis X (FIG. 2B) thatis in parallel to the top surface 205 of the outer ring body 204 suchthat a bottom surface 350 of the inner ring body 206 that was in contactwith an upper surface 203 of the electrostatic chuck 202 can be freshlyprovided as a seating area for a substrate while the planar, unerodedbottom surface 350 now facing upwards and defining areas A and B. Thus,the inner ring body 206 may be used a second time with the originalbottom surface 350 now facing upwards until the outer ring body 204and/or inner ring body 206 have eroded beyond their useful life.

While the ring shape body is shown and described in instantspecification, it is understood that this concept of “flip over andreuse” should not be limited to annular ring shape. Any object withdesired shape or profile is contemplated as long as the object can bedisposed radially inward of the outer ring and flipped into a differentposition to provide a substantially planar seating area for thesubstrate. The outer ring body 204 and inner ring body 206 can be madewith different non-conducting materials to reduce the erosion rate. Theouter ring body 204 can also be coated or made with materials such asceramics, PEEK, polyamides, polyimides, epoxy, etc., which are resistantto corrosive environment and other non conducting materials. Ringassemblies may also be used in dielectric, metal and polysilicon etchand other applications. In conductor etching, the inner ring can becoated with non-conducting material such as ceramic to increase itsservice life.

The two-piece design of the ring assembly 230 enables the outer ringbody 204 to be least sacrificed during the process. The two-piece designof the ring assembly 230 also enables multiple reuse of the outer ringbody 204 and thereby saves the quartz material and reduces the CoO. Thisconcept of protecting inner ring under outer ring, flip over and reusecan also be applied to redesign existing other rings or single rings toincrease service life. This concept may also be extended to chambersprocessing substrate or wafer sizes of any size.

The ring assembly disclosed herein can be used in dielectric etchchambers such as the MxP+, eMxP+, Super-E, EMAX and EMAX CT+ chambersavailable from Applied Materials, Inc., Santa Clara, Calif.Additionally, the ring assembly disclosed herein may also be used inmetal/conductor etch chambers such as the DPS II available from AppliedMaterials, Inc. It is to be understood that the ring assembly discussedherein may be used in chambers sold by other manufacturers as well.

FIG. 4 is a schematic, cross sectional view showing the ring assembly230 illustrating spacing between the inner ring body 206 and outer ringbody 204 according to one embodiment. The spacing (Gap G1 and Gap G2) isthe respective vertical and horizontal separation between the inner ringbody 206 and the outer ring body 204. In regards to Table I, the exampleshown is to ensure avoidance of plasma penetration and is specific toone ring used in dielectric etch chambers. The actual numbers may varyfor other etch rings or the type of the etch applications.

TABLE I Maximum (inches) Minimum (inches) Gap 1 - G1 0.008 0.002 Gap 2 -G2 0.0275 0.0025

FIG. 5A is a schematic cross sectional view of an outer ring body 500 ofa ring assembly 550 according to one embodiment. FIG. 5B is an enlargedview of a portion of the outer ring body 500 illustrated in FIG. 5A. Inone embodiment, the outer ring body 500 may comprise quartz. In anotherembodiment, the outer ring body 500 may comprise clear, fused quartz.The outer ring body 500 may also be fabricated from or coated with othermaterials.

As shown in FIGS. 5A and 5B, the outer ring body 500 includes a mainbody 520 having a generally planar top surface 502 and a generallyplanar bottom surface 504 that is substantially parallel to the topsurface. While not shown here, it is understood that a substrate isgenerally surrounded by the outer ring body 500, and an upper surface ofthe substrate is facing a direction away from the bottom surface 504 ofthe outer ring body 500. The outer ring body 500 also includes anoutside surface 506 that extends between the top surface 502 and thebottom surface 504. The outer ring body 500 also includes an insidesurface 508 that extends between the top surface 502 and the bottomsurface 504. The top surface 502, bottom surface 504, outside surface506 and inside surface 508 collectively comprise the outer ring mainbody 520. The outer ring main body 520 has a height of between about 0.5inches and about 0.75 inches as shown by arrows “C”. The inside surface508 also includes a notch 530 which nests with an inner ring body 600,described below with reference to FIGS. 6A-B. The notch 530 isconfigured to project from the upper portion of the main body 520 andform a substantially horizontal overhang or roof 532 which covers aportion of the area “Q” of the inner ring body 600 when nested. In otherwords, the outside diameter surface 606 (FIG. 6B) of the inner ring body600 is disposed against an inner wall 534 of the outer ring body 500.The inner ring body 600 has a geometry that allows the ring assembly 550to maintain a single sectional profile when the inner ring body 600 andouter ring body 500 are nested, no matter which of the lower or uppersurfaces 602, 604 of the inner ring body 600 is mated with the notch 530of outer ring body 500, as shown in FIG. 3B. In one embodiment, theinside surface 508 is substantially vertical and perpendicular to theroof 532.

In one embodiment, a first annular extension 510 is extended from thebottom surface 504 of the outer ring main body 520. The first annularextension 510 extends in a direction away from the top surface 502 andis disposed at a location spaced from the edge of the outside surface506. In one example, the first annular extension 510 extends from thebottom surface 504 for a distance of between about 0.15 inches to about0.20 inches as shown by arrows “D”. The outside diameter of the firstannular extension 510 is between about 9.75 inches and about 10.1 inchesas shown by arrows “M”. The inside diameter of the first annularextension 510 is between about 9.73 inches and about 9.93 inches asshown by arrows “L”.

A second annular extension 512 may also extend from the bottom surface504 in a direction away from the top surface 502. The second annularextension 512 may abut the inside surface 508. In one embodiment, thesecond annular extension 512 extends from the bottom surface 504 for adistance of between about 0.097 inches to about 0.103 inches as shown byarrows “G”. In one embodiment, the outside diameter of the secondannular extension 512 is between about 9.000 inches and about 9.100inches as shown by arrows “K”. In another embodiment, the insidediameter of the second annular extension 512 is between about 8.300inches and about 8.500 inches as shown by arrows “J”. The first annularextension 510 may extend from the bottom surface 504 by a greaterdistance than the second annular extension 512 of between about 0.070inches and about 0.075 inches as shown by arrows “F”.

A third annular extension 514 may extend from the main body 520 in adirection away from the outside surface 506. An end of the third annularextension 514 forming part of the inside surface 508 may be angledrelative to the roof 532 at an angle of between about 100 degrees andabout 120 degrees, as shown by arrows “E”. The roof 532 forming thebottom of the third annular extension 514 may be at a height of betweenabout 0.25 inches and about 0.30 inches above the bottom of the secondannular extension 512, as shown by arrows “H”. The diameter of the thirdannular extension 514 as measured at the bottom of the third annularextension 514 is between about 7.980 inches and about 8.000 inches, asshown by arrows “I”. It is contemplated that the numbers listed here mayvary depending upon the size of the substrate or chamber.

FIG. 6A is a schematic cross sectional view of the inner ring body 600of the ring assembly 550 according to one embodiment. The inner ringbody 600 and outer ring body 500 as illustrated in FIGS. 6A and 5Aconstitute a ring assembly as discussed and shown previously in FIGS.2A-4. FIG. 6B is an enlarged view of a portion of FIG. 6A. In oneembodiment, the inner ring body 600 may comprise quartz or othermaterial. In another embodiment, the inner ring body 600 may compriseclear, fused quartz. The inner ring body 600 may have an inner diameterof between about 7.730 inches and about 7.740 inches as shown by arrows“N”. The inner ring body 600 may have a height of between about 0.275inches and about 0.300 inches as shown by arrows “P”. The inner ringbody 600 may have a square or rectangular cross-section. Lower and uppersurfaces 602, 604 of the inner ring body 600 are parallel andperpendicular to the center axis of the inner ring body 600. An outsidediameter surface 606 of the inner ring body 600 may be perpendicular tothe surfaces 602, 604 or have another shape which conforms to the insidesurface 508 of the notch 530 below the roof 532 of the outer ring body500 which is configured to nest with the inner ring body 600 when eithersurfaces 602, 604 are disposed adjacent the roof 532 of the outer ringbody 500.

By utilizing a ring assembly that comprises two ring bodies (i.e., innerand outer rings) rather than a single piece, the service life of thering assembly is improved relative to a conventional single piece ring.Particularly, the concept of flip over and reuse of the inner ringreduces the wastage of material. The outer ring that is usedcontinuously for multiple wet cleans until its service life is overfurther reduce the cost of consumables to customers. The usage ofnon-conducting materials also improves the service life of the ringassembly.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A ring assembly, comprising: an inner ring bodyhaving a top planar surface and a bottom planar surface; and an outerring body having a top surface, a bottom surface substantially parallelto the top surface, and an inside surface that extends between the topsurface and the bottom surface, wherein the inside surface has a roofand a portion of the inner ring body is symmetrically nestable under theroof against the outer ring body in two orientations.
 2. The ringassembly of claim 1, wherein the outer ring body further comprises: anoutside surface extending between the top and bottom surfaces; a firstannular extension extending from the bottom surface in a direction awayfrom the top surface and from a location spaced from the outsidesurface; a second annular extension extending from the bottom surface ina direction away from the top surface, the second annular extensionabutting the inside surface; and a third annular extension extendingfrom the outer ring body in a direction away from the outside surface,the third annular extension forming part of the inside surface.
 3. Thering assembly of claim 2, wherein the third annular extension has asurface slanted at an angle relative to the top surface of outer ringbody.
 4. The ring assembly of claim 3, wherein the angle is betweenabout 100 degrees and about 120 degrees.
 5. The ring assembly of claim3, wherein the outside surface has a generally curved surface.
 6. Thering assembly of claim 1, wherein the inner ring body can be flippedover along an axis that is parallel to the top surface of the outer ringbody so that a top surface of a profile of the ring assembly does notchange regardless of the bottom planar surface or top planar surface ofthe inner ring body being disposed against the outer ring body.
 7. Aring assembly, comprising: an outer ring body, comprising: a top surfaceand a bottom surface substantially parallel to the top surface; anoutside surface extending between the top and bottom surfaces; an insidesurface extending between the top and bottom surfaces, the insidesurface having a notch, and the notch having a roof projecting from anupper portion of the outer ring body; a first and second annularextensions extending from the bottom surface of the outer ring body,wherein the first and second annular extensions are parallel andradially spaced from each other; and a third annular extension extendingfrom the outer ring body in a direction perpendicular to the first andsecond annular extensions, the third annular extension forming part ofthe inside surface and having a surface slanted relative to the topsurface and the bottom surface of the outer ring body at an angle abovezero; and an inner ring body disposed radially inward of the outer ringbody, wherein the inner ring body is symmetrically nestable in the notchin two flipped orientations, the inner ring body comprising: a topsurface abutting and projecting inward of the third annular extension;and an outside diameter surface disposed below the third annularextension and abutting the outer ring body.
 8. The ring assembly ofclaim 7, wherein the nested inner ring body and outer ring body have thesame combined sectional profile when the inner ring body is in eitherflipped orientation.
 9. The ring assembly of claim 8, wherein the thirdannular extension partially covers the top surface of the inner ringbody.
 10. The ring assembly of claim 7, wherein the angle is betweenabout 100 degrees and about 120 degrees.
 11. The ring assembly of claim10, wherein the first annular extension and the second annular extensionextend different distances from the bottom surface of the first ringbody.
 12. The ring assembly of claim 7, wherein the second ring bodycomprises quartz or clear fused quartz.
 13. A ring assembly, comprising:an outer ring body having a top surface, a bottom surface opposite thetop surface, an outside surface extending between the top and bottomsurfaces, and an extension extending from the outer ring body in adirection away from the outside surface, wherein the extension has asurface slanted relative to the top surface at an angle above zero; andan inner ring body being surrounded by the outer ring body, wherein theinner ring body is symmetrically nestable against the outer ring body intwo orientations, the nested inner ring body and outer ring body havingthe same combined sectional profile when the inner ring body is ineither flipped orientation.
 14. The ring assembly of claim 13, whereinthe inner and outer ring bodies comprise quartz.
 15. The ring assemblyof claim 13, wherein the inner and outer ring bodies comprisenon-conducting materials.
 16. The ring assembly of claim 13, wherein theangle is between about 100 degrees and about 120 degrees.
 17. The ringassembly of claim 13, wherein the extension radially extends a distanceto partially cover a portion of the inner ring body when the inner ringbody is nested against the outer ring body.